This invention relates to a combinatorial weighing system, particularly a combinatorial weighing method and apparatus for a case where weighing is executed to obtain a target weight value greater than the maximum load of the apparatus, wherein the target value can be weighed out through only a small number of weighing operations.
A combinatorial weighing apparatus has a plurality of weighing machines each consisting of a weighing hopper and a weight sensor associated with the weighing hopper. According to a known combinatorial weighing method using the aforesaid apparatus, combinatorial weighing is carried out by weighing articles which have been introduced into the weighing hoppers of the weighing machines, selecting the combination of weighing, machines (referred to as the "optimum" combination) that gives a total weight value equal to a target weight value or closest to the target weight value within preset allowable limits, discharging only those articles contained by the weighing hoppers of the selected weighing machines, and subsequently replenishing the emptied weighing hoppers with new articles to prepare for the next weighing cycle. The foregoing sequence of steps is repeated to automatically carry out a continuous, highly accurate weighing operation.
In a combinatorial weighing apparatus which employs the foregoing combinatorial weighing method, it is sometimes necessary to weigh out articles in excess of the maximum load of the weighing apparatus. To accomplish this, it is common practice to either (A) divide a target weight value X.sub.a into a number of weight values X1, X2, X3 . . . each of which is less than the maximum load, and then simply repeat the combinatorial weighing operation a plurality of times, or (B) divide the target weight into a number of weight values each of which is less than the maximum load of the apparatus and then, in conducting weighing from the second weighing operation onward, correcting the target weights X2, X3, . . . by any error in the results of the preceding weighing operation.
Weighing method (B) outlined above will now be described in greater detail with reference to the flowchart of FIG. 1. We will assume that the target weight value X.sub.a is 3X grams, and that X1=X, X2=X, X3=X. In order to weigh out 3X grams of the articles, method (B) proceeds in the following fashion:
(1) First, all of the weighing machines are supplied with articles to be weighed.
(2) The weights of the articles fed into the weighing hoppers of the weighing machines are measured (first weight measurement).
(3) Combinations are computed with Xl (=X) grams serving as the target, and the difference between X and Y1, which is the total weight value of the articles contained by those weighing machines that give the optimum combination, is stored in memory as an error E1 (=Y1-X).
(4) The articles are discharged from the weighing machines that give the optimum combination (first discharge operation).
(5) The emptied weighing hoppers of the weighing machines, that is, those that have discharged their articles, are supplied with articles afresh.
(6) The weights of the articles fed into each of the weighing hoppers of the weighing machines are measured (second weight measurement).
(7) Combinations are computed with X2-E1 (=X-E1) grams serving as the target, and the difference between the target value (X-E1) and Y2, which is the total weight value of the articles contained by those weighing machines that give the optimum combination, is stored in memory as an error E2 (=Y1-X). It should be noted that: ##EQU1##
(8) The articles are discharged from the weighing machines that give the optimum combination (second discharge operation).
(9) The weighing hoppers of the weighing machines that have discharged their articles are supplied with articles afresh.
(10) The weights of the articles fed into each of the weighing hoppers of the weighing machines are measured (third weight measurement).
(11) Combinations are computed with X3-E2 (=X-E2) grams serving as the target, and the articles are discharged from the weighing machines that give the optimum combination (third discharge operation). The end result is 3X grams of the articles.
A disadvantage encountered with the above-described target weight dividing method, when weighing out articles to a weight greater than the maximum load, is that combinatorial computations must be performed a considerable number of times to obtain a target weight above the maximum load. Therefore, the method is not suitable for weighing at high speed. Another problem with the foregoing combinatorial weighing method is that there are instances where some weighing machines remain unselected for a prolonged period of time so that the weighing hoppers thereof retain their articles for too long. The reason for prolonged retention of articles in a weighing hopper is that the article batch has a peculiar weight which does not lend itself to selection. If article batches having peculiar weights grow in number because they are unfit for selection, a situation will eventually arise in which no desirable combinations can be obtained. Furthermore, articles such as frozen foods will thaw or spoil if retained in the weighing hoppers for an extended period of time. It is obvious, therefore, that prolonged retention of articles in unselected weighing hoppers is undesirable and should be avoided.